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3b vs 2b

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Old 09-22-2019, 04:28 AM
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340cudaman
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Default 3b vs 2b

I have a 55" aeromarine apache with twin AMPS c/r outdrives connected to a 1:1 gearbox and a gas motor. I have 2b alu 55mm props which gives me 20 actual mph, scale is around 170mph. I'm looking at a pair of 3b 50mm alu.

My question is will they work? I know, at least with planes, you go smaller dia going to 3b from 2b.
Is 50mm going to be about right for my boat? I hate to spend the money if they won't. I'm looking for the scale aspect, speed around 20mph and looks as most real boats have 3b or 4b.

Thanks in advance,
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Old 09-23-2019, 01:58 AM
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I know little about outdrives or gas motors, but I do know a bit about scale and its effects on the conditions that models operate in. Unless the "real" full size boat was something over 330 feet long, scale speed is not 170 MPH. OTOH if the 55" model scales to 1/12, the actual speed of 20 MPH becomes about 70 MPH scale. All to do with doing physics and understanding square roots and not reading boxes on imported toys where the writing is just marks done in ink.
To move a hull forward by its immersed length in a given length of time, that volume of water has to be moved the opposite direction at least the same as the immersed length. For a set level of power, a smaller prop of similar pitch spinning faster will move the water further (and therefore faster), the larger, or more bladed prop will give faster accelleration but with reduced top speed. Finding the best compromise is where performance boating becomes a black art.
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Old 09-23-2019, 04:03 AM
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Default 3b vs 2b

mfr02,
MY boat is scaled after the apache 41' offshore racer. That makes it 1/9 th scale. Reggie Fountain set a world record for deep v offshore top speed at 179mph.
my calculations at 1/9 th scale @ 20mph actual is where I came up with 170 scale mph.

Thanks for the info on the props!

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Old 09-23-2019, 10:45 AM
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The people who write the stuff that appears on boxes think that because a boat might be a particular scale, that the speed of the model multiples by the same linear number. Not so. It multiples by the square root of the scale - 1/9 scale, square root of 9, 3. 3 times 20, you get a 60mph boat. The model, to simulate a boat going 170, wil need to be going almost 60mph. Having a model of a record breaker, and making it perform as the record breaker, involves going through much the same development as the original, proucing the power in the first place, then getting the prop or props to translate the power to moving water. Then keeping it the right end up at that speed. Thats how and why test tanks have been so useful over the years.
Weight is not linear, either - your boat does not weigh 1/9th of the real thing, more like 1/729th. Another point is that record runs happen in very restricted circumstances - for boats, the flattest water possible. Weather also scales, but not in the favour of the model. Nice gentle 10 mph breeze for you, 30mph nearly gale for the model.
I don't know for sure, but guessing that full size props benefit from better detailed (less percentage error) machining, more consistent honing and balancing than is realistically pssible on a model.. Fewer blades means that you get an acceptable result more easiy. Probably. Maybe somebody who knows can chime in.
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Old 09-23-2019, 12:26 PM
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The OP doesn’t tell us much. No mention of the pitch of his props, which of course is what determines the potential speed of his boat. Doesn’t mention anything about the motor either, a converted weedwacker with a top rpm of maybe 10,000 or a race motor with closer to 17,000 rpm. I assume he runs them in a surfacing configuration rather than submerged, perhaps I’m wrong.

The classic formula for boat props is to size a 3-bladed prop to 80% the diameter of the 2-bladed prop, assuming the same blade shape and pitch. It is a rule of thumb, which means it is usually not the optimum. But it’s a place to start. Frankly,
I would not go under 55 mm on a boat that large regardless of blade count.


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Old 09-24-2019, 09:01 AM
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Originally Posted by Got RPM View Post
The OP doesnít tell us much. No mention of the pitch of his props, which of course is what determines the potential speed of his boat. Doesnít mention anything about the motor either, a converted weedwacker with a top rpm of maybe 10,000 or a race motor with closer to 17,000 rpm. I assume he runs them in a surfacing configuration rather than submerged, perhaps Iím wrong.

The classic formula for boat props is to size a 3-bladed prop to 80% the diameter of the 2-bladed prop, assuming the same blade shape and pitch. It is a rule of thumb, which means it is usually not the optimum. But itís a place to start. Frankly,
I would not go under 55 mm on a boat that large regardless of blade count.


.
Thanks, that is what I was looking for, a starting point. I know I'll have to experiment with different props to find what I'm looking for.

Thanks again for the help.

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